Today, not only mobile telephones but also other mobile terminals, such as PDAs (Personal Digital Assistants), notebooks, etc., exchange data with wireless networks via radio interfaces. Typically, a radio base station of a network serves the mobile terminal by routing data received from the terminal through the network towards the recipient, and by transmitting data received from the network side over the radio interface towards the mobile terminal.
The data may be user data such as speech data, media data, streaming data, application data etc., but may also comprise control data (signalling data) associated with, e.g., the establishment of a connection for exchanging user data. In the past, the achievable data rates for data exchange via a radio interface have steadily increased with each new generation of wireless networks. As for mobile networks, so-called 2nd Generation or 2G systems (e.g., GSM systems) provide relatively low data rates in the order of 10 kilobytes per second (kbps), 3rd Generation or 3G systems (e.g., UMTS systems) allow data rates of several 100 kbps and peak rates of up to a few Megabits per second (Mbps). 4th Generation (or simply 4G) systems probably will provide data rates of several 10 Mbps with peak rates of up to 100 Mbps in the downlink (from the base station to the mobile terminal) and 50 Mbps in the uplink (from the terminal to the base station).
To achieve the high data rates of 4G systems, efficient modulation techniques will be implemented in the mobile terminals and in the base stations. Additionally, the higher data rates require a larger frequency bandwidth for each physical channel. In GSM systems, a channel bandwidth of 0.2 MHz is used. In UMTS systems, a channel bandwidth of already 5 MHz is required, and 4G systems will presumably have a bandwidth of up to 20 MHz per channel. 4G standards will allow adjusting the maximum channel transmission bandwidth in steps of (presumably) 1.25 MHz. Typical maximum channel transmission bandwidths of 4G systems may then range from 5 MHz to 20 MHz. The feature of a flexible maximum transmission bandwidth per communication system allows a smooth migration from GSM and UMTS systems to the high data rate of 4G systems, for example by re-using for 4G systems the radio frequency spectrum currently reserved for GSM and UMTS.
As an example for a 4G standard, 3GPP (3rd Generation Partnership Project) responsible for the UMTS standardization proposes a 4G system called LTE (Long Term Evolution) that evolves from the 3G WCDMA (Wideband-CDMA) standard. The UMTS LTE system shall be able to operate on bandwidths spanning from at least 1.25 MHz to at most 20 MHz, supporting micro cells with a radius of 10 meters and peak data rates of up to 100 Mbps therein.
The control procedures performed over the radio interface will in the future also have to take into account the features of variable bandwidth systems, i.e. that mobile terminals and radio base stations are capable of handling different bandwidths (within a maximum bandwidth typically predefined by the communications standard). One of these control procedures that will have to be adapted in this regard is the random access procedure.
A mobile terminal has to perform a random access procedure in order to get access over the radio interface to the wireless network. Prior to the random access procedure, the terminal receives data (only) from a downlink common control channel (DCCH), such a downlink control channel is, for example, the broadcast common control channel in GSM networks.
A DCCH of a base station provides information to all mobile terminals located within the radio cell served by the base station. The signalling transmitted in a DCCH typically relates to information on the actual system, on frequency synchronization, time synchronization and an estimate of the transmit power to be used by the mobile terminals. A synchronization between a mobile terminal and a radio base station has eventually to achieve bit accuracy, i.e. any transmission of the terminal during a particular time slot has to fit into the corresponding time slot at the base station such that none of the transmitted bits exceeds the base station time slot. At least at present, such an accurate time alignment cannot be achieved based only on synchronization information transmitted on the DCCH.
The random access procedure therefore allows the base station to determine the accurate time alignment by measuring the round-trip delay of information sent to the mobile terminal (e.g., in the DCCH) and transmitted back to the base station (in an access request sent by the terminal). As one result of the random access procedure, the base station may transmit a so-called “timing advance” to the terminal, which commands the terminal to shift its transmission scheme (including the timing of the time slots at the terminal) such that the transmissions arrive in the corresponding time slots at the base station with bit accuracy.
To enable the base station to measure the round-trip delay accurately enough, the terminal has to send an “access burst”, which differs from a normal transmission burst by the comparatively long guard period provided to avoid an overlap of the (probably misaligned) access burst received at the base station with bursts received in the neighbouring time slots. Further, for measuring the round-trip delay the product of time length and bandwidth occupied by the access burst has to satisfy a predefined minimum value.
It is to be noted that the terminal typically provides further information with the access burst which, e.g., allows the base station to decide if it actually should grant access to the wireless network. For example, a connection setup reason may be transmitted with the access burst (e.g. ‘emergency call’).
The access burst is sent within a random access channel (RACH). As an example, the RACH in WCDMA systems may be sent in an arbitrary time slot and over the whole available bandwidth. As therefore the random access burst overlays with other transmissions (i.e., the RACH is non-orthogonal to other channels), an accurate control of transmit power is required. Typically, a power ramping procedure is performed, which leads to a delay of the access procedure. As a further disadvantage, the radio base station needs to continuously search for access bursts in all time slots and over the whole bandwidth supported.
In other systems, for example GSM systems, an orthogonal RACH may be provided, which may be achieved by periodically allocating a particular time slot for the random access and the entire available transmission bandwidth. To preserve orthogonality, guard periods have to be included in the time slot due to the timing uncertainties in the uplink. However, with this scheme time and frequency resources are statically assigned to the random access procedure and may only be changed by changing the allocated access time slots, e.g. their periodicity. The lengths of the reserved access time slots cannot be reduced arbitrarily, as the guard period is required, which depends on the cell size.
There is a need for a technique for performing a random access procedure over a radio interface, which allows to flexibly provide time and/or frequency resources to the random access procedure.